1. Field of the Invention
This invention relates to an improved process for preparing a cast film of a thermoplastic resin by an electrostatic pinning technique. According to this process, a cast film of a thermoplastic resin having excellent thickness uniformity and surface smoothness can be easily produced on an industrial scale at high speeds without the undesirable pinner bubble problem and without the trouble of replication of the profile of a quenching surface onto the cast film.
More specifically, this invention pertains to a process for preparing a thermoplastic resin film by extruding a molten thermoplastic film-forming resin in the form of a continuous molten film onto an electrically grounded moving quenching surface, passing the molten film in proximity to but out of contact with at least one electrode which imparts an electrostatic charge to the film across its full width, and withdrawing a solidified film thus formed from said quenching surface.
2. Description of the Prior Art
U.S. Pat. No. 3,223,757 (British Pat. No. 911,528) or British Pat. No. 1,310,609 discloses a process for preparing a thermoplastic resin film by extruding a molten thermoplastic film-forming resin in the form of a continuous molten film onto an electrically grounded moving quenching surface, passing the molten film in proximity to but out of contact with at least one electrode which imparts an electrostatic charge to the film across its full width, and withdrawing a solidified film thus formed from said quenching surface. When the above process using an electrostatic pinning means is operated at a low casting speed, there can be formed a thermoplastic resin film having good thickness uniformity and surface smoothness. If, however, the process is practiced at a casting speed satisfactory for industrial operation, the pinner bubble problem as described, for example, in British Pat. No. 1,302,642 arises. Specifically, the amount of the electrostatic charge per unit area of the quenching film decreases unduly as the casting speed increases. As a result, the electrostatic force of bringing the film into intimate adhesion to the quenching surface becomes insufficient, and air comes between the film and the quenching surface and reduces the uniformity of the quenching effect. In addition, this air produces traces of minute bubbles on the film. The number and size of these bubbles tend to increase as the casting speed increases, and this constitutes a drawback.
In an attempt to overcome this problem, British Pat. No. 1,302,642 proposed an improvement in the aforesaid process, in which the molten film is extruded onto a quenching surface having a surface roughness of at least 15 microinches, r.m.s., and a multiplicity of interconnecting pits. The British patent states that surface roughness in excess of 60 microinches, r.m.s., results in surface irregularities in the finished film which are equal to or greater than the defects caused by the pinner bubbles experienced when using electrostatic pinning with a smooth quenching surface. Furthermore, British Pat. No. 1,302,642 teaches that the quenching surface used in the process of this patent can be prepared by blasting a smooth metallic quenching surface with hard particulate material such as sand, glass beads, shot or aluminum oxide grit having a general particle size of about from 80 to 320 mesh.
Investigations of the present inventors have shown that the cast film of thermoplastic resin obtained by the process of the British patent still has problems to be solved.
For example, base films of high-density magnetic recording media, or optical materials such as photographic materials require films having a high level of thickness uniformity and surface smoothness which are free from fine surface irregularities (orange peel) and do not cause trouble in packaging materials or electrical insulating materials. However, with thermoplastic resin films obtained by electrostatic pinning using the quenching surface disclosed in the British patent which has a surface roughness of at least 15 microinches, r.m.s., and a multiplicity of interconnecting pits, and which is prepared by blasting a smooth metallic quenching surface with hard particulate material, it is difficult to avoid satisfactorily the occurrence of microscopic surface irregularities (orange peel) resulting from replication of the profile of the blasted surface onto the films. It has been found that, for example, for use as high-density magnetic recording media optical materials, these irregularities cause technical trouble. On the other hand, if the surface roughness of the blasted surface is reduced to below 15 microinches, r.m.s., this trouble will be alleviated, but as a matter of course, the pinner bubble problem arises. To avoid this problem, there is no way but to decrease the casting speed, and industrially satisfactory casting speeds cannot be employed.
In the process disclosed in the British patent for preparing a cast film of a thermoplastic resin by electrostatic pinning by using a quenching surface having a surface roughness of at least 15 microinches, r.m.s., and a multiplicity of interconnecting pits prepared by blasting a smooth metallic quenching surface with hard particulate material, the casting speed is at most about 80 m/min. and higher speeds cannot actually be employed because of the pinner bubble problem. In addition, it has been found that even such low casting speeds cannot be maintained for a long period of time, but decrease with time. In the above process, low-molecular-weight compounds contained in the molten thermoplastic film-forming resin tend to adhere to, and build up on, the quenching surface. Presumably because of the blocking of the interconnecting pits by these low-molecular-weight compounds, the pinner bubble problem becomes outstanding with time, and the desired operating speed cannot be maintained. Furthermore, as a result of such blocking, bubbles present between the quenching surface and the molten thermoplastic film-forming resin in the form of a continuous molten film in contact therewith increase, and the uniformity of the quenching effect is reduced.